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Original question:

MUX506: MUX506 voltage

MUX506: MUX input variation

Arya Mohan
Prodigy 140 points
Part Number: MUX506

Hi,

I am using MUX506 in my design. The input to the mux comes after a non-inverting buffer amplifier. My supply is +15V. When I give -10V to channel 1 and +1V to channel 2 , the INPUT at MUX506 was 0.92V at channel 2 and hence output was 0.919V. But when I give -5V to channel 1 and +1V to channel 2, then the INPUT at MUX506 was 0.97 V at channel 2. It is seen that my input voltage at channel 2 depends on input voltage at channel 1. Why this happens and what to do? My MUX input is directly driven from opamp.Do I need to provide a series resistance from opamp output to MUX input?

  • 0
    TI__Intellectual 1465 points

    Hi, Arya,

    Can you give more information on the setup and confirm the following?

    Measurement 1:

    • VS1= -10V
    • VS2= 0.92V
    • Channel 2 ON
    • VD= 0.919V

    Measurement 2:

    • VS1= -5V
    • VS2= 0.97V
    • Channel ? ON
    • VD= ?

    Also, what's the loading condition of the MUX (ie. What load is connected on the MUX output)? 

    Thanks,

    David

  • 0 in reply to David_Wang
    Prodigy 140 points

    Measurement 2:

    • VS1= -5V
    • VS2= 0.97V
    • Channel 2 ON
    • VD= 0.965V

    MUX output is connected to non inverting terminal of an opamp which is configured as voltage follower.

  • 0 in reply to Arya Mohan
    TI__Intellectual 1465 points

    This does look strange. I don't have an explanation. Somehow the output, when CH2 turns on, is loading the input. 

    Is this a DC measurement or is this a temporary spike in voltage when you switch from CH1 to CH2?

  • 0 in reply to David_Wang
    Prodigy 140 points

    It is a DC measurement. Do I have to put an resistor before MUX channel inputs?

  • 0 in reply to Arya Mohan
    TI__Intellectual 1465 points

    Hi, Arya,

    I took some measurement in the lab and didn't notice anything abnormal like what you are seeing on your end. 

    I don't think adding a resistor helps you in this case as this is a DC measurement and the MUX is interfacing a high-impedance load on the output. 

    I am not sure why this issue is happening on your set up. One last thing I could think of is this can potentially be a result of charge injection when you toggle between channels. One way to experiment it is to add additional capacities load on the output (drain) and see if it improves the reading. Since Q= CV and the amount of charge is fixed, adding capacitance should reduce the voltage glitch you see if this is really resulted from the charge injection. 

    Thanks,
    David

  • 0 in reply to David_Wang
    TI__Genius 9132 points

    Hi Arya,

    We haven't heard from you in a while so we will assume your issue was resolved. I will mark this issue as closed but if you have additional questions you may reply here to re open the thread or start a new one. 

    Thanks

    Saminah